In the last ten to fifteen years, intensive efforts and developments have occurred in chemical sensor research and in chemical sensing detection methods and instruments for occupational safety, environmental monitoring, and for processing or quality control. Optical sensors and sensing apparatus have been of particular interest; and the use of optical fibers and optical fiber strands in combination with light energy absorbing dyes for medical, biochemical, and chemical analytical determinations has undergone rapid development.
Conventional optical sensors and optical sensing apparatus, whether or not optical fibers are used, typically employ one or more light energy absorbing dyes which are specific for an analyte of interest and will selectively bind with that analyte. Thus, when light of an appropriate wavelength is introduced to and has been absorbed by the dye, the light energy which is either not absorbed or is returned as an emission is observed and measured by a detection system. The interactions between the light energy conveyed and the properties of the specifically--binding, light absorbing dye--in the presence of one or more ligands or analytes of interest and in the absence of any ligands or analytes whatsoever-provide an optical basis for both qualitative and quantitative determinations. This traditional approach, for both optical and non-optical sensors alike, has therefore been to create highly selective sensors by finding and using specific binding materials. This overall approach consequently results in creating one sensor for each analyte or ligand of interest to be detected. The one analyte/one sensor approach thus has been previously and remains today the overriding guiding principle and axiom for optical chemical sensors and optical chemical sensing apparatus.
It is useful to recognize and appreciate the stringent demands and essential requirements of the traditional one analyte/one sensor approach. These include: (1) each sensor must employ and use one highly selective/specific binding agent for binding and reaction with a single analyte or ligand of interest in a sample; (2) the sensor relies and depends upon the energy signal generated by the selective binding agent as the means for detecting and determining the presence of the single analyte or ligand in the sample; (3) the approach requires that for detection of multiple analytes or ligands, a series of different selective binding agents with individual and different binding specificities are used together as multiples concurrently or in sequence; and (4) the specific binding and signal generation of the sensor can be accomplished using a variety of different binding agents including colorimetric or fluorescent dyes, selective polymer films, or biological receptors such as enzymes and antibodies. In each instance, one sensor must be created for the detection of each analyte or ligand of interest.
In comparison, it will be noted that nature has created a biological sensing system which is markedly different both in structure and function from the man-made traditional chemical sensor approach. For example, the mammalian olfactory system is an in-vivo sensor for vaporous odors which is not matched by any artificially synthesized sensor to date in detection limit and discriminatory power. Vapor odor reception is an interaction between olfactory receptor cells and the vapor molecules. In short, the odor is "sensed" by sensory neurons in the olfactory epithelium, followed by the formation of a neuronal activity pattern which consists from multiple different responses of receptor cells to the one odor. The activity pattern of affected sensory neurons is projected to the olfactory bulb; and the response patterns are then transmitted to the other various brain regions for recognition and identification. This system is unique because, rather than having one receptor for each specific molecule, a variety of different sensory neurons are involved; and each of them recognizes one or more properties of the odor. As a result, a large population of different sensory neurons will respond to a given odor; but each neuron responds differently--thereby giving rise to an odor-specific output response pattern. It is believed that the neuronal circuitry of the olfactory bulb recognizes and identifies this odor-specific output pattern through processing with its circuits.
The concept of employing chemical sensors using pattern recognition systems analogous to those of the mammalian olfactory system has been modestly explored by a number of different research laboratories; and the few detection systems using such an analogous pattern recognition approach today are popularly referred to as "smart sensor systems", or "odor-sensing systems", or "electronic noses". Representative of these research investigations and systems are the following publications: Abe et al., Anal. Chem. Acta. 194:1-9 (1987) and 215:155-168 (1988); Carey et al., Anal. chem. 58:149-153 (1986), Anal. Chem. 58:3077-3084 (1986), Sens. Actuators 9:223-224 (1986), Anal. Chem. 59:1529-1534 (1987); Ema et al., Sens. Actuators 18:291-296 (1989); Abe et el., Anal. Chem. Acta. 215:155-168 (1988); Stetter et al., Anal. Chem. 58:860-866 (1986); Gardner, J. W., Sens. Actuators B 4:109-115 (1991); Muller, R. and E. Lang, Sens. Actuators 9:39-48 (1986); Muller, R., Sens. Actuators B 4:35-39 (1991); Ballantine et el., Anal. Chem. 58:3058-3066 (1986); Rose-Pehrrson et el., Anal. Chem. 60:2801-2811 (1988); and Grate et al., Sens. Actuators B 3:85-111 (1991).
The use of chemical sensors with pattern recognition capabilities have to date taken two non-optical structural formats: the use of surface acoustic wave (SAW) or bulk acoustic wave (BAW) sensors; and the use of piezoelectric sensors. The bulk acoustic wave sensors and surface acoustic wave devices are piezoelectric crystals which have been coated on the external surface with a polymer or a high boiling liquid. BAW and SAW devices are chemical sensors which rely on mass changes, oscillator circuitry and electronic controls to operate the various subsystems and to collect and process the data received. Such detection systems are well described by the following publications: Grate et el., Anal. Chem. 65:1868-1881 (1993); Patrash, S. J. and E. T. Zellers, Anal. Chem. 65:2055-2066 (1993); Rose-Pehrrson et el., Anal. Chem. 60:2801-2811 (1988); Carey et el., Anal, Chem, 59:1529-1534 (1987); Zellers et. el., Sens. Actuators 12:123-133 (1993); Grate et el., Anal. Chem. 60:869-875 (1988); and Grate et al., Anal. Chem. 64:610-624 (1992).
In comparison, piezoelectric chemical sensors are quartz crystal electrodes coated with a polymeric film. The use of such piezoelectric sensors to investigate fragrances and the nature of human reactions to different odors is exemplified by Yokoyama, K. and F. Ebisawa, Anal. Chem, 65:673-677 (1993).
Insofar as is presently known to date, therefore, while the concept of pattern recognition as an approach for detection of analytes has been explored as a potential alternative to traditional chemical sensors and chemical sensing systems which require one sensor for each analyte or ligand to be detected, all of these prior investigations have been electrically based and rely upon changes in electrical signals as the means for detection and evaluation. In particular, no optical sensor or optical sensing system has ever been envisioned or constructed which would operate to detect multiple spectral responses or evaluate them as spectral recognition patterns. Instead, the conventional guiding principle and requisite axiom of one specifically binding sensor for each analyte or ligand to be detected remains rigidly in force, as demonstrated by the most recent innovations in optical sensors and optical detection systems conventionally. Accordingly, were an optical sensor and detection system developed which would be only semi-selective in its binding and reaction characteristics such that a single dye reagent would provide a variety of different spectral responses for multiple analytes and ligands in a manner which was both accurate and reproducible, such a novel optical innovation and detection system would be recognized as a major pioneering advance and achievement over conventional detection instruments and methods.